The present invention relates to injection-molded closures, and more particularly to a thin walled, injection-molded closure and container usable with various contents including frozen contents in a container.
Many products, including frozen or refrigerated product, are stored in plastic containers having a complementary plastic lid or closure which engages and seals the container and the contents therein. Some of the very cold products are subjected to a xe2x88x9240xc2x0 F. blast freezing process and then are subjected to a drop in temperature and often are kept at 0xc2x0 F. in the frozen foods departments of a grocery store or the like. These cold temperatures make the plastic of the closures and containers more brittle and more likely to fail under tests to which the closed containers are subjected, such as drop tests, while the containers are cold and full of the contents. For cold closures being drop tested, the stress is concentrated at sharp corners on the closure, which tends to fail there. Typically, conventional closures or lids are made from polyethylene such as low density polyethylene, linear flow density polyethylene and high density polyethylene plastic. For injection-molded polyethylene closures, the lower limit for the wall thickness is usually above 0.024 inch thick. While the closures made of these materials having thick walls of 0.020 inch and greater performed adequately, for some applications there is a desire to reduce that thickness of the wall and to make the container less costly because of having less plastic therein.
Conflicting with the desire to reduce the wall thickness when using one of the polyethylene plastics in an injection mold closure, is a desire to have a central panel of the closure be flat and planer for printing or staying out of contact with the container contents. That is, when the polyethylene closure panel is molded to be less than 0.020 inch thick, there is a problem maintaining the central panel in a flat horizontal plane for printing or for aesthetic reasons. Although polypropylene plastic as well as polyethylene has been used in the manufacture of injection-molded lids, polypropylene is not generally used in cold applications because the polypropylene plastic has a generally more brittle characteristic at cold temperatures than the polyethylene plastic.
Linear flow polyethylene is commonly used to injection mold closures because it flows adequately within narrow lid cross sections in the mold and has good strength characteristics particularly for low temperature applications of frozen foods or the like. Polypropylene actually has a better melt index in that it flows better in thin cross-section parts than does polyethylene, but polypropylene recrystalizes faster than does polyethylene. Because of these and various other shortcomings of polypropylene, it has not been used in injection mold closure lids having a thin cross section, e.g., of 0.020 inch or less. Polyethylene is more flexible than polypropylene, making it easier to strip closure portions that overlie a portion of the mold steel during a stripping and ejecting of a molded closure from the mold.
Another general shortcoming of polypropylene plastic in its use to manufacture injection-molded lids is that unlike the polyethylene plastic, after molding, polypropylene does not continue to shrink in, resulting in what is called xe2x80x9ctoe-inxe2x80x9d of the bottom of the skirt wall. That is, in polyethylene closures, the outer skirt contracts upon cooling of the injected plastic to form a lesser diameter at the bottom of the skirt to define a toe-in angle, which is the angle between the vertical and the taper of the lid skirt. Toe-in occurs in polyethylene lids and is used advantageously in the nesting and stacking of lids one-on-another. More specifically, one manner of stacking such closures made of polyethylene is to provide an upstanding stacking ring on the top of a closure and a xe2x80x9ctoe-inxe2x80x9d centering engagement between the tapered upper skirt of the upper closure with a portion of the lower closure. Another form of stacking with a conventional polyethylene lid is the use of stacking ribs, which are ribs formed in the peripheral rim portion for engagement with another rim. However, the use of such stacking rings and stacking ribs adds considerably more plastic to these injection-molded, polyethylene closures.
These kinds of containers and closures are used with automatic filling and capping equipment to fill the containers and to place the closures on the filled plastic containers. Automatic closure handling equipment is also used to feed the closures for printing on the plastic closures after they have been molded. The closures are stored for use in such handling equipment in vertical stacks; and it is important that the space between adjacent skirts on adjacent closures be substantially uniform and centered because a pair of mechanical fingers are usually inserted into the space between lower feeding rings on the lower edges of the skirts to remove the lowermost closure from the stack. Therefore, it is important that the adjacent closures in the stack are not askew or tilted with respect to another, resulting in a larger air gap on one side between the skirts, and a smaller or no gap on the opposite side of the closures in the stack. Further, a partial vacuum should not be formed in an air space between adjacent, stacked closures that would cause the closures to stick together and impede the feeding of the closures. The failure of a closure to feed properly can cause production interruption or possibly equipment damage and is to be avoided. Thus, it will be seen that it is important that the closures, when stacked, are level and centered on the stack and have uniform spacing between adjacent feeding rings on the lower ends of the peripheral skirts of the closures.
Another form of closure that is commonly used particularly with dairy products and the like is a thermo-formed closure, which is made from a sheet of plastic such as polyethylene by a die forcing the plastic into the desired plug configuration. A large number of closures are formed simultaneously in the sheet and then the sheets are cut to form individual closures. The thermo-formed closures have their edges later rolled to form a closure rim having a dependent skirt in a secondary operation. Despite efforts trying to maintain close tolerances for thermo-formed closures, it is found that it is difficult to keep the thermo-formed closures precisely shaped and stacked for use in the automatic equipment. The thermo-formed closures are usually less expensive and contain less polyethylene plastic than injected-molded, polyethylene closures. For the same dairy application, the injected-molded, polyethylene lids have some wall portions of about of 0.024 inch thick as well as stacking ribs; while the thermo-formed lids often are only about 0.014 inch thick. Thus, there is a need for a new and improved injection-molded closure which has thinner walls, uses less plastic to compete with thermo-formed closures, which can be stacked and centered easily for use with automatic handling equipment and yet, which has sufficient rigidity to pass the strength drop test and rigidity to keep a central panel substantially flat and planar for printing or the like.
In accordance with the present invention, an injected, molded plastic closure, particularly for use with cold products, is provided with a thin wall, for example, with good centering and stacking capabilities. This is achieved by the use of a polypropylene injection-molded lid that has a flared, peripheral, depending skirt for stack alignment and centering and has a pair of spaced surfaces for stacking without the use of plastic ribs or an upstanding stacking ring, which use a lot of plastic.
In the illustrated and preferred embodiment of the invention, the thin wall, polypropylene closure has a thickness of less than 0.020 inch while a typical, injection-molded polyethylene closure will have walls with portions of at least 0.023 or 0.024 inch thick; and because of the use of stacking ribs and stacking rings, the polyethylene closure will use much more plastic than the closure of this invention.
In the preferred embodiment of the invention, the stacking of polypropylene closures is done without a toe-in angle for the skirt, as in polyethylene closures that have a toe-in of the skirt, and without a stacking ring that adds more plastic to the closure. This is achieved by the use of a pair of radially-spacing stacking supports and nested, engaged lower skirt portions for self-centering of the closures in the stack. The preferred centering is by flared upper and lower skirt portions being spaced by a small air gap, e.g., 0.002 inch, if perfectly centered, but engaging when not centered to center the closures with respect to one another. This is unlike the toe-in angle centering of polyethylene lids where there is no air gap.
Also, in the preferred embodiment of the invention, the upper closure is supported on the lower closure in a stack by a pair of stacking supports that are radially spaced from one another. The first inner stacking support is formed at the bottom of a substantially vertically-extending wall or panel joining the central panel to the top panel or peripheral rim portion of the closure. The outer stacking support comprises an underside of a bead or retention shoulder for retaining the closure on the container abutting a rim ledge or surface on the closure therebelow. Stack alignment and centering is obtained by a flared, lower skirt on the bottom of a peripheral skirt for engaging a similarly-contoured area on the upper skirt wall of an adjacent closure. Thus, the stacking and centering is obtained without the use of additional plastic being added to mold an upstanding stacking ring and without the toe-in from the polyethylene, or without the use of additional plastic being added to mold stacking ribs also heretofore used with polyethylene lids for stacking.
In accordance with an important aspect of the invention, the closures may be formed with vents to vent an air space between adjacent central panels of stacked closures to prevent the formation of a vacuum in this air space that would cause the closures to stick together. In one embodiment of the invention, the vents are achieved by providing a series of spaced lugs on the inner stacking support with the lugs being spaced from each other to allow air to flow through the spaces between adjacent lugs. In the embodiment of the invention having an annular retention ring for retaining an informational disk, the vents are formed by raised and lowered edges on the upper end of the retention ring to define spaced air vents to allow air to flow between a central panel of an upper closure and the supporting upper end of the annular retention ring of the closure therebelow. Thus, closures may, in accordance with the invention, be vented when stacked.
In accordance with another embodiment of the invention, the closure is provided with a flat, substantially horizontal, outer annular surface for engaging a foil or membrane seal secured across the top of the container at a matching, substantially horizontal, outer annular surface on the container. This closure will have the flared skirt and matching angled wall at the top of the skirt for centering and will have a second point of contact for stacking at or adjacent to the flat, foil engaging surface. Thus, the present invention provides a lightweight closure made of polypropylene that can be used with containers sealed with a foil or membrane.
In accordance with a still further embodiment of the invention, a lightweight, polypropylene closure is provided with a centering ring that projects upwardly and inwardly from the top of the closure to retain a disk of advertizing material, or the like, on the top central panel of the closure. This closure uses the top edge of the centering ring as the inner area of contact with another stacked closure along with the flared skirt and matching angled wall at the top of the skirt for centering and stacking of a stack of closures. Thus, there is provided a lightweight closure with a centering ring that uses the stacking and centering features of this invention.